Supplemental Condensed Consolidating Financial Information (Schedule Of Condensed Consolidating Balance Sheets) (Details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2010	Dec. 31, 2009
Assets
Cash and cash equivalents	$ 82,345	$ 93,593	$ 56,966	$ 61,138
Restricted cash	350,966
Receivables	426,116	252,320
Inventories	350,201	235,616
Other	179,037	153,385
Total current assets	1,388,665	734,914
Property, plant and equipment, net	7,726,456	3,308,892
Other	1,036,324	836,963
Total other assets	1,036,324	836,963
Total assets	10,151,445	4,880,769	4,890,540
Liabilities and Stockholders' Equity
Accounts payable	286,437	198,216
Accrued expenses and other current liabilities	400,811	258,133
Current maturities of debt and short-term borrowings	428,610	70,997
Total current liabilities	1,115,858	527,346
Long-term debt	3,773,923	1,538,744
Asset retirement obligations	412,324	334,257
Accrued pension benefits	36,047	49,154
Accrued postretirement benefits other than pension	87,626	37,793
Accrued workers' compensation	65,027	35,290
Deferred income taxes	863,970
Other noncurrent liabilities	258,261	110,234
Total liabilities	6,613,036	2,632,818
Redeemable noncontrolling interest	11,032	10,444
Stockholders' equity	3,527,377	2,237,507
Total liabilities and stockholders' equity	10,151,445	4,880,769
Parent/Issuer [Member]
Assets
Cash and cash equivalents	9,350	13,713	24,396	54,255
Restricted cash	74,814
Receivables	51,241	31,458
Other	29,418	29,575
Total current assets	164,823	74,746
Property, plant and equipment, net	11,268	9,817
Investment in subsidiaries	8,492,557	4,555,233
Intercompany receivables	(2,160,682)	(1,807,902)
Note receivable from Arch Western	225,000	225,000
Other	445,302	481,345
Total other assets	7,002,177	3,453,676
Total assets	7,178,268	3,538,239
Liabilities and Stockholders' Equity
Accounts payable	26,716	10,753
Accrued expenses and other current liabilities	86,198	75,746
Current maturities of debt and short-term borrowings	131,750	14,093
Total current liabilities	244,664	100,592
Long-term debt	3,321,528	1,087,126
Asset retirement obligations	622	873
Accrued pension benefits	13,087	20,843
Accrued postretirement benefits other than pension	14,888	14,284
Accrued workers' compensation	15,042	15,383
Deferred income taxes	(20,679)
Other noncurrent liabilities	50,707	51,187
Total liabilities	3,639,859	1,290,288
Redeemable noncontrolling interest	11,032	10,444
Stockholders' equity	3,527,377	2,237,507
Total liabilities and stockholders' equity	7,178,268	3,538,239
Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	2,134	64	64	64
Restricted cash	276,152
Receivables	134,803	12,740
Inventories	177,614	85,196
Other	133,887	102,375
Total current assets	724,590	200,375
Property, plant and equipment, net	6,257,368	1,800,578
Intercompany receivables	818,816	508,624
Other	577,022	344,698
Total other assets	1,395,838	853,322
Total assets	8,377,796	2,854,275
Liabilities and Stockholders' Equity
Accounts payable	164,243	65,793
Accrued expenses and other current liabilities	164,682	31,123
Current maturities of debt and short-term borrowings	296,860
Total current liabilities	625,785	96,916
Long-term debt	1,101
Asset retirement obligations	105,428	32,029
Accrued pension benefits	4,333	4,407
Accrued postretirement benefits other than pension	49,734
Accrued workers' compensation	43,140	13,805
Deferred income taxes	884,649
Other noncurrent liabilities	171,002	22,135
Total liabilities	1,885,172	169,292
Stockholders' equity	6,492,624	2,684,983
Total liabilities and stockholders' equity	8,377,796	2,854,275
Non-Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	70,861	79,816	32,506	6,819
Receivables	241,653	210,075
Inventories	172,587	150,420
Other	15,732	21,435
Total current assets	500,833	461,746
Property, plant and equipment, net	1,457,820	1,498,497
Intercompany receivables	1,341,866	1,299,278
Other	14,000	10,920
Total other assets	1,355,866	1,310,198
Total assets	3,314,519	3,270,441
Liabilities and Stockholders' Equity
Accounts payable	95,478	121,670
Accrued expenses and other current liabilities	151,512	153,217
Current maturities of debt and short-term borrowings		56,904
Total current liabilities	246,990	331,791
Long-term debt	451,294	451,618
Note payable to Arch Coal	225,000	225,000
Asset retirement obligations	306,274	301,355
Accrued pension benefits	18,627	23,904
Accrued postretirement benefits other than pension	23,004	23,509
Accrued workers' compensation	6,845	6,102
Other noncurrent liabilities	36,552	36,912
Total liabilities	1,314,586	1,400,191
Stockholders' equity	1,999,933	1,870,250
Total liabilities and stockholders' equity	3,314,519	3,270,441
Eliminations [Member]
Assets
Receivables	(1,581)	(1,953)
Total current assets	(1,581)	(1,953)
Investment in subsidiaries	(8,492,557)	(4,555,233)
Note receivable from Arch Western	(225,000)	(225,000)
Total other assets	(8,717,557)	(4,780,233)
Total assets	(8,719,138)	(4,782,186)
Liabilities and Stockholders' Equity
Accrued expenses and other current liabilities	(1,581)	(1,953)
Total current liabilities	(1,581)	(1,953)
Note payable to Arch Coal	(225,000)	(225,000)
Total liabilities	(226,581)	(226,953)
Stockholders' equity	(8,492,557)	(4,555,233)
Total liabilities and stockholders' equity	$ (8,719,138)	$ (4,782,186)